1. Field of the Invention
The present invention relates to the production of thin films. In particular, the invention concerns a method of growing a thin film onto a substrate, in which method the substrate is placed in a reaction chamber and it is subjected to surface reactions of a plurality of vapour-phase reactants according to the ALD method to form a thin film. The invention also concerns apparatus for implementing the method.
2. Description of Related Art
Conventionally, thin films are grown out using vacuum evaporation deposition, Molecular Beam Epitaxy (MBE) and other similar vacuum deposition techniques, different variants of Chemical Vapor Deposition (CVD) (including low-pressure and metallo-organic CVD and plasma-enhanced CVD) or, alternatively, the above-mentioned deposition process based on alternate surface reactions, known in the art as the Atomic Layer Deposition, in the following abbreviated ALD, formerly also called Atomic Layer Epitaxy or “ALE”. Equipment for the ALD process is supplied under the name ALCVD™ by ASM Microchemistry Oy, Espoo, Finland. In the MBE and CVD processes, besides other variables, the thin film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform surface smoothness of the thin films manufactured using these methods, the concentrations and reactivities of the starting materials must be kept equal on one side of the substrate. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, the possibility of mutual reactions between the reagents is always imminent. Herein arises a risk of microparticle formation already in the infeed lines of the gaseous reactants. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. However, the occurrence of premature reactions in MBE and CVD reactors can be avoided, e.g., by heating the reactants not earlier than only at the substrates. In addition to heating, the desired reaction can be initiated with the help of, e.g., plasma or other similar activating means.
In MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the thin film growth rate in the ALD process is controlled by the substrate surface properties, rather than by the concentrations or other qualities of the starting material inflows. In the ALD process, the only prerequisite is that the starting material is provided in a sufficient concentration for film growth on the substrate.
The ALD method is described, e.g., in FI Patents Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI Patents Nos. 97,730, 97,731 and 100,409 are disclosed some apparatus constructions suited for implementing the method. Equipment for thin film deposition are further described in publications Material Science Report 4(7), 1989, p. 261, and Tyhjiötekniikka (title in English: Vacuum Techniques), ISBN 951-794-422-5, pp. 253–261.
In the ALD method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon. Thus, typically, an ALD method comprises the steps of:                feeding vapour-phase reactants into an reaction chamber in the form of vapour-phase pulses repeatedly and alternately; and        causing said vapour-phase reactants to react with the surface of the substrate to form a thin film compound on the substrate.        
According to the conventional techniques known from FI Patent Specification No. 57,975, the saturation step is followed by a protective gas pulse forming a diffusion barrier that sweeps away the excess starting material and the gaseous reaction products from the substrate. The successive pulses of different starting materials and the protective gas pulses forming diffusion barriers that separate the successive starting materials pulses from each other accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. To the function of the process it is irrelevant whether they are the gases or the substrates that are kept in motion, but rather, it is imperative that the different starting materials of the successive reaction steps are separated from each other and arranged to impinge on the substrate alternately.
Even if the above-described arrangement is rather reliable, it has some disadvantages. For instance, the cross sections and shapes of piping in practical reactor constructions vary between, e.g., the inlet manifold and the substrates, whereby the thickness and shape of the diffusion barrier become difficult to control and the starting materials may become carried over into contact with each other. The diffusion wall may also become destroyed in the nozzles feeding the vapor-phase reactant to the substrates, in gas mixers or at other discontinuity points of the piping. The laminarity of gas inflow may also become disturbed by a too tight bend in the piping.
Intermixing of starting materials in flow systems cannot be prevented simply by keeping the gas volumes apart from each other, because mixing may also occur due to adherence of molecules from a starting material pulse on the apparatus walls or discontinuities thereof, wherefrom the molecules may then gain access with the molecules of the successive starting material pulse.
As a solution to the above problems we have earlier designed an ALD process and apparatus wherein each starting material pulse is individually driven through the piping and reaction space of the apparatus isolated from the other pulses (U.S. Pat. No. 6,015,590). According to that invention, the gas volume of the reaction space is purged between two successive vapor-phase pulses essentially entirely, which means a purging efficiency of at least 99%, advantageously 99.99%. All the reacting gas, in practice the entire gas volume filled with the vapor-phase reactant, is purged from the reaction chamber between the successive pulses. Thus, the reactant pulses of different starting materials remain isolated from each other, whereby no mixing of the reactants can occur.
The above-mentioned process is quite efficient. However, there still exists a need of alternative solutions to the intermixing problem. This relates to situations in which the purging/evacuation process is not available or when it is desired further to reduce the concentration of the residues of the previous pulse before feeding the following one into the reaction chamber.